The present invention relates to a resistance-type liquid level measuring apparatus installed in a fuel tank, for detecting a liquid level by a resistance value of a resistor corresponding to a contact position of a movable contact interlocked with a float floating on the surface of a liquid.
In a vehicle traveling by a fuel such as gasoline or methanol, a fuel tank for containing the fuel is mounted, and an engine is driven by fuel supply from this. A fuel gauge indicating the residual quantity of such fuel is installed in a dashboard in front of a driving seat, and a driver confirms this to judge lack of fuel and carries out refueling.
As a fuel gauge for indicating the residual quantity of fuel in a fuel tank as stated above, a cross coil-type instrument using a pointer for indication, a stepping motor-type instrument, a liquid crystal indicator using a bar graph or a digital numerical value for indication, a fluorescent display tube, or the like is used. As a detector for detecting the residual quantity of fuel in a tank, a liquid level sensor is generally known in which a resistance-type sensor, which has a simple structure and is inexpensively constructed, is installed in a fuel tank, and a driver in a driving seat can always confirm the fuel residual quantity indicated by this fuel gauge during the driving, and can judge the necessity of refueling according to a distance to a destination.
In the detection of a fuel residual quantity by a fuel gauge made of such a resistance-type sensor, a movable contact is angularly rotated by an arm coupled with a float floating according to a liquid level, or a contact position is vertically moved by a contact fixed to a ring float, and a resistance value corresponding to the liquid level is obtained through a voltage at a connection position to many conductor electrodes connected with a resistor on an insulating substrate. However, since the position of the contact is changed in a state where it is immersed in the fuel such as gasoline or methanol, and since an electric current is applied to the contact portion, there is a possibility that a problem of contact abrasion or contact fault arises at the contact portion, and it has been improved by the improvement of a contact material. As disclosed in, for example, Japanese Utility Model Publication No. Hei. 4-1682, it is known that such a conductor electrode is made of a mixture of AgPd (silver palladium) powder and glass, and is obtained by mixing Ag (silver) powder, palladium (Pd) powder, and glass powder to form a paste, printing it on an insulating substrate, drying this, and then, firing. Ag (silver) has a low electric resistance and is excellent in conductivity, however, when used in a fuel, it is degraded or corroded by, for example, sulfur, moisture, alcohol, or the like in the fuel, and causes defective continuity.
Especially in a fuel such as bad gasoline, an Ag (silver) component of an electrode or a contact is sulfurized by sulfur in the fuel, and silver sulfide is deposited on the electrode or the contact surface, so that electric resistance between contacts is increased to influence an electric current flowing between the contacts. In a detection system of voltage dividing specification as disclosed in, for example, Japanese Utility Model Publication No. Sho. 60-23709, in which a movable contact is grounded or this movable contact is used as an output terminal, an actual divided voltage by a movable contact of a variable resistor can not be obtained by the influence of the resistance between contacts due to such sulfide, and there is such a problem that an error is produced in the indication of a fuel quantity as a fuel gauge. That is, from an E point (low region: empty side) at which a resistance value is at a maximum position to an F point (full tank: full side) corresponding to a minimum position, especially at the F point, electric resistance between contacts is influenced by silver sulfide, and there occurs such a phenomenon that the indication of the fuel gauge does not accurately indicate the F point though full tank refueling is carried out.
By such phenomenon, in a pointer-type instrument, a rather lower side from the F point scale position of a dial is indicated, and in a bar display by a plurality of segments in an electronic indicator such as a liquid crystal indicator, a segment corresponding to the F point does not operate, and segments at the lower side are displayed, so that the indication lacking reliability is produced such that in spite of a full tank, the F point is not indicated. A similar minus indication phenomenon occurs also at the E point side of the low region, however, in this case, the E point is indicated even if some fuel remains, and accordingly, such a situation as running out of gas does not occur and there is no problem in practical use. However, there arise a problem that a distrust is produced at refueling if the indication remains minus at the F point.
Further, such a phenomenon also occurs in a structure used for, for example, a fuel tank of a motorcycle, in which a main tank and a sub-tank are provided, resistance-type sensors are respectively installed in the tanks, and these resistance-type sensors are connected in series with each other to obtain the sum of residual quantities of fuel in both the tanks, and an indication error by the increase of contact resistance due to silver sulfide has a greater influence as the sum of the two resistance-type sensors.
In order to elucidate the mechanism of silver sulfide generation as stated above, the present inventor prepared many resistance-type sensors, immersed them in a liquid containing sulfur, measured a voltage between contacts due to generation of silver sulfide, that is, a voltage drop generated by deposited silver sulfide, and investigated and studied the influence exerted on the indication of a fuel gauge. In the resistance-type sensors of experimental objects, a conductor electrode material contained AgPd and a contact material was an alloy containing CuNi (copper nickel) as its main ingredient, or an alloy containing CuNiZn (copper nickel zinc) as its main ingredient or an alloy containing it. A change of a voltage drop VS between contact points due to the deposition of silver sulfide was measured under the conditions of a power supply voltage V=5 v, a resistance value of a voltage dividing resistance RO=120 xcexa9, and as a resistance value of a resistor R1, 130 xcexa9 at the E point position to 13 xcexa9 at the F point position. As a result, a phenomenon with saturated voltage occurred in which although the voltage drop VS was gradually increased with the degree of deposition of silver sulfide, a further increase was not seen at approximately 0.4 v to 0.6 v. Further, in order to confirm such phenomenon, experiments were repeated in which the deposition of silver sulfide was advanced and the power supply voltage V was changed. As a result, it was found that the voltage became constant at approximately 0.4 v to 0.6 v, and a further increase did not occur. This is because a current application mechanism through silver sulfide deposited on the electrode surface operates as a semiconductor to cause a function just like a diode, and it appears that a constant voltage drop phenomenon of 0.4 v occurs irrespective of the quantity of the deposition and irrespective of the magnitude of the power supply voltage. Incidentally, even if AgPd (silver palladium) alloy, AgCu (silver copper) alloy, AgNi (silver nickel) alloy or the like is used as the contact material, the voltage drop VS to the deposition of silver sulfide becomes also about 0.4 v by the same function.
With respect to such voltage drop VS=0.4 v, in the case where a detection voltage VO is obtained at a connection point between a voltage dividing resistance RO and a resistor R1 under a power supply voltage V=5 v, the detection voltage is VO=(5xe2x88x920.4)xc3x97R1/(RO+R1)+0.4, and when a voltage equivalent to a full tank of a liquid level is VF, and a voltage equivalent to a low region is VE, VF=(5xe2x88x920.4)xc3x9713/(120+13)+0.4=0.85 v, and VE=(5xe2x88x920.4)xc3x97130/(120+130)+0.4 =2.79 v. A change width corresponding to an indication range of such detection voltage VO becomes about 2 v, and if the change of 2 v is converted and indicated within the range of from the F point to the E point of the scale of an indicating instrument, an error of the voltage drop VS=0.4 v due to silver sulfide reaches 25% of the whole indication angle. With respect to the output voltage in a normal state where the voltage drop VS by the increase of contact resistance due to silver sulfide is hardly generated, from the output voltage VF=5xc3x9713/(120+13)=0.49 v equivalent to the full tank to the output voltage VE=5xc3x97130/(120+130)=2.6 v corresponding to the low region, especially at the time of the full tank, the indication is greatly shifted to the side of the E point from the F point by the voltage drop of 0.4 v, and becomes impermissible as an indication error on the scale. For example, there is a problem that although a full tank is caused at the time of refueling, the indication position is much lower than the F point.
The present inventor paid attention to such a voltage drop phenomenon due to silver sulfide, and changed an approach in such a direction that a conductor electrode material was not changed or transformed as a measure of suppressing the generation of silver sulfide, or a material measure was suppressed to the minimum, and the generation phenomenon of silver sulfide as stated above was accepted, however, if the influence of the voltage drop phenomenon on the indication of a fuel gauge was suppressed to such a degree that it could be neglected in practical use, a resistance-type sensor as stated above, which was used as a set including an indicating instrument, became satisfactory. Besides, an improvement was made under the thought that if the voltage indication error by the voltage drop, such as 0.4 v, due to silver sulfide at the F point, where a practical influence was highest, was suppressed, a practical problem could be resolved.
A resistance-type liquid level measuring apparatus in the present invention is provided on an attachment plate 3 attached to an opening of a fuel tank 2, and is constructed, at a lower side of the attachment plate 3, as a resistance-type sensor 1 constituted by an insulating substrate 6 made of ceramic on which a conductor electrode 4 made of an alloy containing silver, for example, AgPd (silver palladium) alloy and a resistor 5 are formed by printing, and a slider 8 (movable contact) provided with a contact portion 7 which slides on the conductor electrode 4. The conductor electrode 4 is made of plural lines of electrode patterns (a plurality of conductor electrodes 4) arranged in a fan shape (substantially parallel) along a rotation trajectory of the contact portion 7 and at suitable intervals on the insulating substrate 6, and the resistor 5 is made by firing a printed layer containing, for example, ruthenium oxide as its main ingredient, and is adhered so as to continuously cover a part of each of the plural lines of the electrode patterns constituting the conductor electrode 4.
The vertical movement of a liquid level is transmitted to the slider 8 through a float 9 and an arm 10, the contact portion 7 slides on the conductor electrode 4 so that the conductor electrode 4 on the insulating substrate 6 comes in contact with the contact portion 7, a resistance value of an effective resistor between a position of the contacted conductor electrode 4 and one end side is changed, and a value of a current flowing between the contact portion 7 and the conductor electrode 4 is determined. The change of this current value is outputted as a detection signal of a voltage value through an output terminal. Here, resistance values of the respective resistors are set such that a resistance value RO of a voltage dividing resistance 11 is RO=420 xcexa9, a resistance value RE of the resistor 5 at the time when the liquid level is at a low region is RE=447 xcexa9, and a resistance value RF at the time of a full tank is RF=13 xcexa9. A power supply voltage V at this time is 10 v.
By this, when the resistance-type sensor 1 is used in a fuel containing sulfur, relative to a voltage drop VS=0.4 v due to silver sulfide deposited between the slider 8 (movable contact) and the conductor electrode 4, when a detection voltage VO equivalent to a full tank of a liquid level is VF, and a voltage equivalent to a low region is VE, a voltage output of |VFxe2x88x92VE| greater than 4 v (volt) is obtained, and the voltage drop VS generated by silver sulfide becomes 15% or less of the detection voltage VO, and can be suppressed to be sufficiently low as an indication error.
Besides, as another embodiment of the resistance-type sensor 1, among the respective conductor electrodes P of the electrode patterns arranged in the fan shape and substantially parallel with each other to constitute the conductor electrode 4 on the insulating substrate 6, a resistance value of the resistor 5 positioned between a conductor electrode PF corresponding to the F point equivalent to the full tank of the liquid level and a lower side conductor electrode P1 adjacent thereto is set to be larger than a resistance value between other conductor electrodes P positioned at the lower side, the sum of voltage differences xcex94VF respectively between detection voltages VF and V1 at the connection point when the movable contact 8 comes in contact with the conductor electrode PF and the conductor electrode P1 is 0.4 v (volt) or larger, and the indication characteristic at an indication unit 102 is made such that the F point is indicated until a voltage becomes one obtained by adding 0.4 v (volt) or larger to the detection voltage VO at the conductor electrode PF when sulfurization does not occur, whereby with respect to the indication at least at the F point position, even if the voltage drop due to silver sulfide is generated, it is not indicated as an error, and such a disadvantage that the F point is not indicated is resolved though a full tank state is caused by refueling.
Further, in addition to the error absorption structure at the F point, if the foregoing structure is also used in which when the detected voltage VO equivalent to the full tank of the liquid level is made VF and the voltage equivalent to the low region is made VE, the voltage output of |VFxe2x88x92VE| greater than 4 v (volt) is obtained so that the voltage drop VS generated by silver sulfide becomes 15% or less of the detection voltage VO, the error at the F point is absorbed, and an indication error in the whole indication region can also be suppressed to be sufficiently low in practical use.